Various freeze processes have been developed to separate water from brackish water as well as contaminated aqueous waste streams containing dissolved or suspended salts. See, for example, the U.S. Pat. Nos. of Ashley et al 3,070,969; 3,477,241; 3,501,924; Ganiaris 3,620,034; Johnson 3,664,145 and Ogman 4,091,635.
More recent freeze processes employ a vertical shell and tube freeze exchanger in which an ice slurry is formed. The ice slurry is discharged from the bottom of the freeze exchanger into an ice slurry receiver-crystallizer. The ice slurry is subsequently sent to an ice separator-washer Such apparatus is disclosed in the U.S Pat. Nos. of Engdahl 4,314,455 and Husain et al 4,750,333.
The described apparatus is suitable for freeze concentrating waste water streams having a relatively low concentration of precipitable salts. It is inherent in the freeze concentration process that as the amount of ice formed increases that the concentration of salts in the solution increases, thereby causing them to precipitate. Even though such precipitates carry over with the ice to the washer-separator they can be handled satisfactorily up to a reasonable amount. However, high strength waste water, i.e. waste water that contains a considerable fraction of precipitable salt, is not readily processed in such apparatus because the precipitable salts carry over into the ice separating, washing and melting stage. This adversely affects the product water quality because the precipitable salts that are carried over with the ice into the washing stage and into the melter redissolve into the product water, thereby increasing the salt content of the product water. It is crucial to obtaining product water of acceptable purity for discharge to the environment that the precipitable salts be separated. A need accordingly exists for apparatus and methods for concentrating waste streams having a substantial concentration of contaminants.